The invention relates to a process for the production of alkali metal-, in particular sodium- and potassium- and of ammonium peroxodisulfate by anodic oxidation of an aqueous solution containing an alkali metal- or ammonium sulfate or -hydrogen sulfate.
The production of alkali metal- and ammonium peroxodisulfate by anodic oxidation of an aqueous solution containing the corresponding sulfate or hydrogen sulfate and the recovery of the salt from the anolyte by crystallisation is known.
According to DE-PS 27 57 861, sodium peroxodisulfate is produced with a current efficiency of 70 to 80% in an electrolytic cell with a cathode protected by a diaphragm and a platinum anode, by electrolysing a neutral aqueous anolyte solution with a starting content of 5 to 9 wt. % sodium ions, 12 to 30 wt. % sulfate ions, 1 to 4 wt. % ammonium ions, 6 to 30 wt. % peroxodisulfate ions and a potential-increasing agent, known as a promoter, such as in particular thiocyanate, using a sulfuric acid solution as the catholyte at a current density of at least 0.5 to 2 A/cm2. After crystallising out and separating of peroxodisulfate from the anolyte, the mother liquor is mixed with the cathode product, neutralised and returned to the anode. The disadvantages of this process are firstly the necessity of using a promoter to reduce the formation of oxygen, secondly the need for a higher current density and thus a high anode potential to obtain an economically viable current efficiency and thirdly the problems associated with the production of the platinum anode of obtaining an acceptable current efficiency for industrial purposes and a long anode lifetime.
A filter-press type electrolytic cell for the production of peroxo compounds, including ammonium peroxodisulfate, sodium peroxodisulfate and potassium peroxodisulfate is known from EP-B 0 428 171. Here, platinum films applied to a valve metal with a hot isostatic press are used as anodes. A solution of the corresponding sulfate containing a promoter and sulfuric acid is used as the anolyte. This process too has the problems referred to above.
In the process of DE-OS 199 13 820, peroxodisulfates are produced by anodic oxidation of an aqueous solution containing neutral ammonium sulfate. To produce sodium- or potassium peroxodisulfate, the solution obtaining [sic]from anodic oxidation, which contains ammonium peroxodisulfate, is reacted with sodium- or potassium hydroxide solution; after crystallisation and separation of the corresponding alkali metal peroxodisulfate, the mother liquor is recycled in mixture with the catholyte produced during electrolysis. Here too, electrolysis is carried out in the presence of a promoter at a platinum electrode as the anode.
Although for many decades peroxodisulfate has been recovered on an industrial scale by anodic oxidation at a platinum anode, there are still serious disadvantages associated with these processes:
Polarisers, also called promoters, must always be added to increase oxygen overvoltage and improve current efficiency; Oxidation products of these promoters penetrate the anode waste gas as toxic substances and must be removed by gas washing.
The anodes, the entire surface of which is normally coated with platinum, always require a high current density. This results in a high current loading of the anolyte volume, the separator and the cathode which necessitates additional measures, for reducing the cathodic current density by three-dimensional structuring and activation. There is also high thermal loading of the labile peroxodisulfate solution. To minimise this loading, structural measures must be taken and the cooling costs also increase. The electrode surface must be limited as a result of the restrictive dissipation of heat, and this increases the installation costs per cell unit. To overcome the high current loading, electrode support materials with good heat transfer properties must generally also be used, and these are susceptible to corrosion and costly.
In Electro Chemical and Solid-State letters, 3(2) 77-79 (2000), P. A. Michaud et. al. disclose the production of peroxodisulfuric acid by anodic oxidation of sulfuric acid using a diamond thin-film electrode doped with boron. This document discloses that such electrodes have a higher overvoltage for oxygen than platinum electrodes, but it is not possible to deduce from this document whether diamond thin-film electrodes doped with boron can also be used for industrial production of ammonium- and alkali metal peroxodisulfates. It is known that sulfuric acid on the one hand and hydrogen sulfates, in particular neutral sulfates, on the other behave very differently during anodic oxidation. In spite of the higher overvoltage of oxygen at diamond electrodes doped with boron, the main subsidiary reaction in addition to anodic oxidation of sulfuric acid is the formation of oxygen and additionally ozone.
The object of the present invention is to demonstrate an industrial process for the production of ammonium- and alkali metal peroxodisulfates, in which the disadvantages of the known processes are at least reduced. It was found, surprisingly, that it is possible to produce ammonium- and alkali metal peroxodisulfates with high current efficiency, by using as the anode a diamond thin-film electrode doped with a tri- or pentavalent element. Surprisingly, promotors can be omitted completely and electrolysis can be carried out at low current density, which produces further advantages.
Accordingly, the present invention relates to a process for the production of a peroxodisulfate of the series ammonium-, sodium- and potassium peroxodisulfate, by anodic oxidation of an aqueous electrolyte containing a salt of the series ammonium-, sodium- and potassium sulfate and/or the corresponding hydrogen sulfate, in an electrolytic cell comprising at least one anode, one cathode and one anolyte area, this being separated by a separator from a catholyte area, or adjoining a gas diffusion cathode, characterised in that a diamond film mounted on a conductive carrier and made conductive by doping with a tri- or pentavalent element is used as the anode and no promoter is added to the anolyte. The subclaims relate to preferred embodiments of this process.
When producing the conductive diamond film which acts as an anode it is doped with one or more tri- or pentavalent elements until it has been doped with a sufficient quantity to ensure adequate conductivity. The doped diamond film is thus an n-type conductor or a p-type conductor. It is useful for the conductive diamond film to be mounted on a conductive carrier material, which can be selected from the series silicon, germanium, titanium, zirconium, niobium, tantalum, molybdenum and tungsten and carbides of these elements. Alternatively, a conductive diamond film can also be applied to aluminium. Particularly preferred carrier materials for the diamond film are silicon, titanium, niobium, tantalum and tungsten and carbides of these elements.
A particularly suitable electrode material for the anode is a boron-doped diamond thin film on silicon.
The diamond electrodes can be produced by two special CVD processes (chemical vapor deposition technic [sic]). They are microwave-plasma CVD and high-wire CVD. In both cases, the gas phase which is activated to plasma by microwave radiation or thermally by hot wires, is formed from methane, hydrogen and optionally other additives, in particular a gaseous compound of the doping agent. By using a boron compound, such as trimethyl boron, a p-type semiconductor is formed. Using a gaseous phosporus compound as a doping agent produces an n-type semiconductor. Depositing the doped diamond film on crystalline silicon produces a particularly dense and pore-free filmxe2x80x94a film thickness of approximately 1 xcexcm is normally sufficient. As an alternative to depositing the diamond film on a crystalline material, it can also be deposited on a self-inhibiting material such as titanium, tantalum, tungsten or niobium. For production of a particularly suitable boron-doped diamond film on a silicon monocrystal, see the previously mentioned article by P. A. Michaud.
Ammonium and sodium peroxodisulfate can be produced in conventional electrolytic cells, which can also be collected in the form of a filter pack. Here, the anode and cathode areas are separated by a separator. The separator can for example be a conventional porous material produced from an oxidic material, but is preferably an ion exchange membrane. Materials which are already known in the prior art, such as lead, carbon, tin, zirconium, platinum, nickel and alloys thereof, preferably lead, are suitable as cathodes.
According to an alternative embodiment of the electrolytic cell, the cathode is in the form of a gas diffusion electrode, and the cathode is provided with an oxygen-containing gas. Electrolysis can thus be carried out at considerably lower cell voltages, which saves a considerable amount of energy. In this case there is no need for a separate anolyte circuit or a microporous or ion exchanging separator, which simplifies the whole process considerably and represents a significant technical improvement on all processes known hitherto.
According to a preferred embodiment, the electrolytic cell comprises a circuit for the liquid anolyte and another circuit for a liquid catholyte. According to the invention the anolyte can be sulfur-acid or neutral and contains ammonium- and/or alkali metal cations, sulfate- and /or hydrogen sulfate anions, preferably also peroxodisulfate anions, but no polariser. In principle, the anolyte composition can correspond to those referred to in the documents of the prior art cited at the beginning, the difference being however, that no promoter is added or is otherwise present.
For the production of ammonium peroxodisulfate, the starting anolyte preferably contains, per litre, 300 to 500 g ammonium sulfate and 0 to 0.2 mol sulfuric acid per mol ammonium sulfate. A substantially neutral starting anolyte is preferred. In this case the catholyte is a sulfur-acid ammonium sulfate solution. It is useful to carry out the anodic oxidation at an anodic current density in the range 50 to 1000 mA/cm2, preferably 400 to 900 mA/cm2. Ammonium peroxodisulfate is recovered in the known way from an anolyte stream washed out from the anolye circuit, processing preferably comprising vacuum crystallisation and separation of the crystals from the mother liquor. The anolyte mother liquor is recirculated for electrolysis after increasing the content of ammonium sulfate or ammonium hydrogen sulfate which can be done by mixing it with the catholyte produced and adding a base as required.
Sodium peroxodisulfate can be recovered either immediately after anodic oxidation of an anolyte containing sodium hydrogen sulfate, the anolyte preferably containing 500 to 600 g NaHSO4 per liter. In this case an aqueous solution containing 300 to 400 g H2SO4 per liter and 300 to 500 g Na2SO4 per liter is used as the catholyte. Alternatively sodium peroxodisulfate can also be recovered in the known way, by reacting an anolyte containing ammonium peroxodisulfate from anodic oxidation of ammonium sulfate or ammonium hydrogen sulfate, with sodium hydroxide solution, then crystallising out the sodium peroxodisulfate and separating it from the mother liquor, see DE-OS 199 13 820 and DE-PS 27 57 861 for examples of the relevant embodiments.
In a similar way to sodium peroxodisulfate, potassium peroxodisulfate can also be produced using a solution containing potassium sulfate and ammonium sulfate or potassium hydrogen sulfate.